Cardiac arrhythmias, commonly known as irregular heart beats or racing hearts, are the result of various physical defects in the heart itself. One such defect is an extraneous strand of muscle fiber in the heart that provides an abnormal short-circuit pathway for electric impulses traveling through the heart tissue. This accessory pathway often causes the electric impulses that normally travel from the upper to the lower chamber of the heart to be fed back to the upper chamber, causing the heart to beat irregularly and therefore inefficiently pump blood.
Another common type of cardiac arrhythmia is ventricular tachycardia (VT), which may be a complication resulting from a heart attack or from a temporary reduction of blood supply to an area of heart muscle. VT often is caused by a tiny lesion, typically on the order of one to two millimeters, that is located close to the inner surface of the heart chamber. That lesion is often referred to as an "active site", because it does not fire in sequence with the rest of the heart muscle. VT causes the heart's normal rhythmic contraction to be altered, thereby affecting heart function. A typical symptom is rapid, inefficient heart beats.
Non-surgical procedures such as management with drugs are favored in the treatment of cardiac arrhythmias. However, some arrhythmias are not treatable with drugs. For example, drug therapy to combat VT is typically successful in only 30 to 50 percent of patients. Because of this low success rate, another conventional remedy is to perform a surgical procedure. According to these procedures, various incisions are made in the heart to block conduction pathways and thus divide the atrial area available for multiple wavelet reentry in an effort to abolish the arrhythmia. Alternatively, an automatic implantable cardioverter/defibrillator (AICD) can be surgically implanted into the patient, as described in U.S. Pat. No. 4,817,608 to Shapland et al. While these surgical procedures can be curative, they are associated with increased morbidity and mortality rates, and are extremely expensive. Even the use of an AICD requires major surgical intervention. Moreover, patients of advanced age or illness often cannot tolerate such invasive surgery.
Minimally invasive techniques have been developed which are used to locate cardiac regions responsible for the cardiac arrhythmia, and also to disable the short-circuit function of these areas. According to these techniques, electrical energy shocks are applied to a portion of the heart tissue to ablate that tissue and produce scars which interrupt the reentrant conduction pathways. The regions to be ablated are usually first determined by endocardial mapping techniques. Mapping typically involves the percutaneous introduction of a diagnostic catheter having one or more electrodes into the patient, passing the diagnostic catheter through a blood vessel (e.g. the femoral vein or aorta) and into an endocardial site (e.g., the atrium or ventricle of the heart), and inducing a tachycardia so that a continuous, simultaneous recording can be made with a multichannel recorder at each of several different endocardial positions. When a tachycardia focus is located, as indicated in the electrocardiogram recording, it is marked by means of a fluoroscopic image so that cardiac arrhythmias at the located site can be ablated. An ablation catheter with one or more electrodes can then provide electrical energy to the tissue adjacent the electrode to create a lesion in the tissue. One or more suitably positioned lesions will create a region of necrotic tissue to disable the malfunction caused by the tachycardia focus.
Conventional catheter ablation techniques have utilized catheters with a single electrode fitted at its tip as one electrical pole. The other electrical pole is conventionally provided by a backplate in contact with a patient's external body part to form a capacitive coupling of the ablation energy source (DC, laser, RF, etc.). Other ablation catheters are known in which multiple electrodes are provided.
Ablation is carried out by applying energy to the catheter electrodes once the electrodes are in contact with the cardiac tissue. The energy can be, for example, RF, DC, ultrasound, microwave, or laser radiation. When RF energy is delivered between the distal tip of a standard electrode catheter and a backplate, there is a localized RF heating effect. This creates a well-defined, discrete lesion slightly larger than the tip electrode (i.e., the "damage range" for the electrode), and also causes the temperature of the tissue in contact with the electrode to rise.
Often, to overcome cardiac arrhythmias such as atrial flutter and atrial fibrillation, it is necessary to create a long, continuous lesion (i.e., a linear lesion). One conventional ablation procedure for creating these linear lesions is commonly referred to as a "drag" method, in which an ablation catheter carrying one or more ablation electrodes is manipulated through a patient's blood vessels and to a desired location within the patient's heart. One or more of the electrodes is manipulated into contact with the heart tissue. Ablation energy is then delivered to the electrode(s), causing them to heat up and scar the adjacent tissue to create a lesion which is typically slightly larger than the surface area of the electrode contacting the tissue (the electrode's damage range). After the electrode has ablated the adjacent tissue, the clinician then manually moves the catheter a selected amount by pulling on the catheter shaft so that the electrode(s) are then aligned, and in contact, with different tissue, and ablation energy is again delivered to the electrode(s) to ablate that tissue. By continuing this procedure, the clinician attempts to create a continuous, linear lesion to block an aberrant pathway.
However, this method of dragging the catheter shaft has a number of disadvantages. For example, once the portion of the catheter shaft carrying the ablation electrode is making good tissue contact, it is undesirable to move the catheter shaft, because of the risk of losing the tissue contact.
Others have attempted to overcome this problem by using a relatively long, cylindrical electrode mounted over the catheter shaft. The long electrode can create longer lesions without requiring that the electrode (and thus the catheter shaft) be moved. However, using long electrodes also has significant drawbacks, one being that an elongated electrode detracts from the flexibility of the catheter, such that the catheter may not be able to assume a desired curve due to the straightening effects of the electrode(s).
Accordingly, it will be apparent that there continues to be a need for a device for performing ablations which facilitates the creation of linear lesions. In addition, there exists the need for a device which does not require the surgeon to physically drag the catheter shaft to create a linear lesion. The instant invention addresses these needs.